What type of pleco are you talking about? A common gibby/pteryg? Or a bristlenose? Or a fancy pseuda/lepora? A small common would probably leave it alone until it get's big enough to be opportunistic to eat the shrimp. A bristlenose would likely leave it alone. The other carnivorous type of plecos may actively seek to prey on the shrimp.
However, if you have lots of floating plants, and the shrimp are the type that like to hide in the floating plants, my understanding is that you probably will have enough shrimp to replace any that get eaten.